Ernie Hubbard decided to research more about kale after finding that "health fanatics" - people who exercised, ate healthily, didn't smoke or drink - were visiting their doctors about complaints such as chronic fatigue, skin and hair issues, arrhythmias, gluten sensitivity and other digestive troubles.

Initially, nobody could pinpoint what the problem was. But after taking urine samples from the patients, Hubbard discovered that many of them registered high levels of thallium and cesium.

He then discovered that the cruciferous family of vegetables - kale, cabbage, broccoli and cauliflower - absorb thallium from the soil.

Putting two and two together, he realised there was most definitely a link.

And while Kale only contains small amounts of the metal, if large amounts are consumed on a daily basis, this can cause health issues such as fatigue, heart problems, nausea and hair loss.

One of Hubbards' patients, a 52-year-old vegetarian who exercised for two hours each day and ate plenty of greens, was complaining of being fatigued and having brain fog - her hair had also begun to fall out.

Hubbard asked her what her favourite vegetable was and she replied that it was kale and cabbage, which she consumed daily.

Her urine samples confirmed that she had worrying amounts of thallium in her system - in fact, the levels were seven times more than what's considered a safe amount.

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30 Superfoods To Try In 2016

30 Superfoods To Try In 2016

1

of

30

Flaxseeds

Health benefits: Good for the heart, thanks to omega-3 essential fatty acids and lignans, as well as colon-cleansing fibre (of which it contains soluble and insoluble types), according to WebMD.
How to eat them: On top of cereal, in yogurt.

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Anna Daniels, a spokesperson for The British Dietetic Association tells HuffPost UK Lifestyle that kale has been heralded as a superfood because it's a rich source of vitamins and minerals: per 100g, Kale has 3.1g fibre, 3.4g protein, 33 calories and is a rich source of carotene and folate.

"Thallium is toxic to mammals and a few studies have highlighted the potential transfer of thallium into soil and thus into the vegetables that are grown in these soils," she explains.

"Yet little is understood in regard to transfer to the crop, which would also differ by crop and also by region due to the soil type."

"What we know today is that the health benefits of eating kale outweigh the risks to the individual consuming this green leafy vegetable as part of a healthy balanced diet," she adds.

"There is no recommended amount of kale to eat in your diet, but 2-3 portions per day would be perfectly acceptable."

According to BDA, consuming vast amounts of the same vegetable could cause toxicity.

"Often the general public may hear of a new superfood and start to eat more of it, which is fine so long as this isn't in excess," says Daniels.

"However, all fruits and vegetables are 'super' in being that they contain differing levels of vitamins and minerals which are good for us."